Seats for children who weigh more than 65 pounds - a growing part of the car seat market, partly because of the increase in childhood obesity - are not held to any government safety requirements. Seats for smaller children and infants are regulated only for their effectiveness in front-end collisions.

Problems with developing child dummies are also a key reason why seats for all children have no federal requirements for effectiveness in side-impact, rear-end and rollover collisions, car seat experts said.

Parents are confronted with a barrage of safety seat choices for children of all sizes. More than 100 models for infants, toddlers and older children are on the market, according to CarseatBlog.com, which is written by parents and car seat experts who monitor the industry. Many parents say they find little information about seats beyond what they cull from private testing organizations, such as Consumer Reports magazine and the Insurance Institute for Highway Safety.

"I think everyone is a little unclear about it, especially the booster seat issue," said Rosemary Berger of Rockville, as she watched her 4-year-old son, Ryan, at the play area in Bethesda's Westfield Montgomery shopping mall. "It's hard to understand what's required and what's been tested."

Berger said she bought a backless booster seat for Ryan after he reached 40 pounds. "But I'm not convinced yet whether he's safe in that," she said.

Sara Keleher of Bethesda said she did plenty of Web research before choosing seats for her 2-year-old and 2-month-old daughters. Still, she said she was not aware that federal regulations do not cover all seats or all types of crashes.

"I'm under the assumption that there's some federal guidelines that companies have to meet," Keleher said.

Car seat manufacturers "self-certify" that their seats meet the safety standards that do exist. The NHTSA tests 75 to 90 models each year, said Ronald Medford, the agency's deputy administrator. Those that do not comply with federal rules are recalled.

The NHTSA only tests for crash protections that are regulated. That leaves parents to rely on manufacturers' assurances for the higher weight seats and for side-impact protections, seat-belt fit and other potential injury factors.

Delay in oversight

Safety experts say a lack of funding for researching and developing lifelike child test dummies has caused the NHTSA's oversight of safety seats to lag years behind in a highly competitive industry that evolves to meet demand.